What if the renegotiated contracted is actually worded heavily in favour of FDJ, especially in the event that Barca instigate his sale before the deferred wages are paid? What if it says Barca must pay every euro of earned salary that has been tacked onto future earnings if the club sell him? Maybe FDJ's camp have actually been incredibly smart in the renegotiation and have Barca over a barrel... something like that would explain FDJ maintaining he doesn't want to leave, even if in reality he is open to, or even actively wants the move. It would explain Barca's DoF broadly saying they need to sell big players because of FFP whilst talking about FDJ, then being publicly corrected by Laporta who keeps pushing the 'no need to sell' line. It would explain United sitting quietly in the wings and just waiting for them to sort out their mess.
It could be that any severance payment made to FDJ counts as salary on Barca's books and given how precarious their wage bill is supposed to be, maybe this really fecks up their accounts somewhere. If they can't afford to pay his salary and can't afford to pay his severance on the stated terms without breaching either a football or financial law, that would also explain everyone's current stance.
There are so many possibilities and so little actual information to go on but I still think it would surely be unlikely that an agent or lawyer at this level of football could be so negligent as to let his client sign such a weak contract.